Sextant

Question 2

What is a sextant?

Answer 2

A tool to measure the angle between two objects (e.g., celestial body & horizon).

Question 3

What is Perpendicular Error?

Answer 3

Index mirror not perpendicular to frame.

Check: Hold sextant sideways, see if true & reflected arc align. Correction: Adjust screw on top of index mirror.

Question 4

What is Side Error?

Answer 4

Horizon mirror not perpendicular to frame.

Check: Arc at 0°, sight horizon – true & reflected horizons won’t align. Correction: Adjust screw furthest from frame on horizon mirror.

Question 5

What is Index Error?

Answer 5

Index & horizon mirrors not parallel at 0°.

Check: Arc at 0°, horizon should line up. Correction: Adjust horizon mirror screw OR apply correction mathematically.

Question 6

How do you apply Index Error mathematically?

Answer 6

If “on the arc” → subtract. If “off the arc” → add.

Mnemonic: “If it’s on, take it off. If it’s off, put it on.”

Question 7

What is Graduation Error?

Answer 7

Inaccurate cutting of arc/micrometer graduations.

Question 8

What is Collimation Error?

Answer 8

Telescope not parallel to sextant’s plane.

Question 9

What is Centering Error?

Answer 9

Pivot of index arm not at true centre of arc.

Question 10

What are Optical/Prismatic Errors?

Answer 10

Shade glasses/mirrors not optically flat (faces not parallel).

Question 11

What does a noon sight give you?

Answer 11

Latitude.

Question 12

Formula for Latitude (LAT) at meridian passage?

Answer 12

Same name, LAT > DEC → LAT = DEC + (90° – ALT). Same name, DEC > LAT → LAT = DEC – (90° – ALT). Opposite names → LAT = (90° – ALT) – DEC.

Question 13

How is longitude found from a noon sight?

Answer 13

By timing Local Noon, then comparing with GHA of the Sun.

Question 14

What is a sextant used for?

Answer 14

To measure the angle between two objects, usually a celestial body and the horizon.

Question 15

What’s the sextant’s principle of operation?

Answer 15

Uses reflection from two mirrors → angle measured = double the angle moved by index arm.

Question 16

What is Perpendicular Error?

Answer 16

Index mirror not perpendicular to frame.

Question 17

How is Perpendicular Error checked and corrected?

Answer 17

Hold sextant sideways, reflected & true arc should align. Correct with index mirror screw.

Question 18

What is Side Error?

Answer 18

Horizon mirror not perpendicular to frame.

Question 19

How do you check and correct Side Error?

Answer 19

At 0°, true & reflected horizons should line up. Correct with horizon mirror screw furthest from frame.

Question 20

What is Index Error?

Answer 20

Index & horizon mirrors not parallel when arc at 0°.

Question 21

How is Index Error checked and corrected?

Answer 21

At 0°, true & reflected horizon must align. Correct with horizon mirror screw or apply mathematically.

Question 22

How do you apply Index Error mathematically?

Answer 22

On the arc → subtract; Off the arc → add.

Question 23

What is Graduation Error?

Answer 23

Inaccurate cutting of sextant arc or micrometer drum.

Question 24

What is Collimation Error?

Answer 24

Telescope not parallel to plane of sextant.

Question 25

What is Centering Error?

Answer 25

Index arm pivot not exactly at centre of arc.

Question 26

What is Prismatic Error?

Answer 26

Shade glasses or mirrors not optically flat/parallel.

Question 27

What are common sextant sights taken for navigation?

Answer 27

Meridian passage (noon sight), morning/evening star sights, Polaris, HSA/VSA for coastal position fixing.

Question 28

What is a Meridian Passage (Noon Sight) used for?

Answer 28

To determine latitude.

Question 29

How is latitude calculated at meridian passage?

Answer 29

LAT = DEC ± (90° – ALT), depending on same/opposite names of LAT & DEC.

Question 30

How is longitude determined at noon sight?

Answer 30

From the exact time of Local Noon compared with GHA of the Sun.

Question 31

What is a Vertical Sextant Angle (VSA) used for?

Answer 31

To find distance off by measuring angle between top and base of object of known height.

Question 32

What is a Horizontal Sextant Angle (HSA) used for?

Answer 32

To fix position using measured angles between three objects.

Question 33

What is a Station Pointer?

Answer 33

A plotting instrument used to lay off sextant angles for a three-point fix.

Question 34